Marrow culture studies will be performed using the methylcellulose assay for detection of committed erythroid precursors (CFU-E and BFU-E) in patients with polycythemia vera. Erythroid precursors will be characterized by density gradient analysis. Populations of buoyant and dense CFU=E and BFU-E will be prepared by discontinuous gradient techniques. The populations will be assayed for erythropoietin responsiveness and cycling status by thymidine suicide. These techniques will attempt to confirm the presence of normal and abnormal clones of committed erythroid precursors in polycythemia vera and to segregate them by their density characteristics. Serial studies will be performed to detect the disappearance of clones of normally responsive precursors. The precursors will be evaluated for responsiveness to burst promoting activity (BPA) and the patients will be evaluated for the production of BPA. Continuous marrow culture techniques will be adapted for human studies and used as an assay for pluripotential stem cells. Density gradient separation will be used to segregate coexisting clones of normal and abnormal pluripotential stem cells from patients with polycythemia vera. Bone morrow in patients with polycythemia vera will be examined for the presence of inhibitors which can cause the suppression of normal clones and allow the selective poliferation of abnormal clones.